This invention relates to an organoleptically improved, water-soluble polydextrose containing 0.3 mol or less (e.g., 0.3 to 0.001 mol %) of bound citric acid, to a process therefor, and to foods containing same. The improved, water-soluble polydextrose of this invention is prepared by reducing the bound citric acid content of unimproved, water-soluble polydextrose, preferably by passing the unimproved, water-soluble polydextrose through one or more specified resins. Bound citric acid is also referred to herein as citric acid bound in the form of ester groups. These citrate esters are primarily dibasic, and so generally retain a measure of acidity. While the process of the invention coincidentally reduces free citric acid to less than 0.1 mol %, free citric acid can be added back where acidity is desired.
As used herein, the expression "water-soluble polydextrose" (also known as polyglucose or poly-D-glucose) specifically refers to the water-soluble polydextrose prepared by melting and heating dextrose (also known as glucose or D-glucose), preferably with about 5-15% by weight of sorbitol present, in the presence of a catalytic amount (about 0.5 to 3.0 mol %) of citric acid. Water-soluble polydextrose is an item of commerce which, as an approved food additive, is defined in the Food and Drug Section of the Code of Federal Regulations (21 C.F.R. 172.841). In its unimproved form, it is also described by Rennhard, U.S. Pat. No. 3,766,165, which claims, inter alia, a "[w]ater-soluble highly branched poly[dextrose] wherein the linkage of 1.fwdarw.6 predominates, having number average molecular weight between about 1,500 and 18,000 and containing from about 0.5 to 5 mole percent of [citric]acid ester groups . . . ", i.e., water-soluble polydextrose characterized by its content of from about 0.5 to 5 mol % of bound citric acid; and by Rennhard, U.S. Pat. No. 3,876,794, which claims various foods containing same. According to Rennhard, water-soluble polydextrose is preferably prepared using 0.5-5 mol percent of citric acid as catalyst. However, since Rennhard's use of about 6 mol percent of citric acid produced more than two thirds undesired insoluble polydextrose, we prefer use of citric acid at a level in the range of about 0.5 to 3 mol percent. The most preferred range is about 0.7 to 1.3 mol percent, approximating the 1% by weight noted in the C.F.R., cited above, Rennhard also specified optional use of about 5-20% (preferably 8-12%) by weight of sorbitol in the polymerization. The narrower range approximates the 10% by weight of sorbitol also noted in the C.F.R., cited above.
However, as variously noted in the art [Torres in U.S. Pat. No. 4,622,233; Goff et al., J. Food Science, vol. 49, pp. 306-307 (1984); Lim et al., J. Food Science, vol. 54, pp. 625-628 (1989)] Rennhard's polydextrose possesses a slight bitter taste which limits the breadth of its use in foods.
Torres believed that the bitter taste of Rennhard's polydextrose was due to the presence of anhydroglucose. While that compound has not been ruled out as one of the factors in the bitter taste, we have now surprisingly found that bound citric acid (i.e. the 0.5 to 5% mol % of citric acid ester groups in Rennhard's polydextrose) is the most important factor in causing said bitter taste.
Rennhard generally suggested the use of ion exchange as a method of reducing the acidity of his polydextrose; e.g., at column 6, lines 48-50 of U.S. Pat. No. 3,766,165. Three types of basic ion exchange resins are available for this purpose, viz: Types I and II strong base anion resins, and weak base anion resins. Type I resins, which contain quaternized amine functional groups are the the most strongly basic and have the greatest affinity for weak acids such as carboxylic acids. However, not all operating conditions for use of Type I resins are effective in preparing the improved polydextrose of this invention. Use of a Type I strongly basic exchange resin outside of the conditions disclosed by this invention leads to polydextrose with inferior taste.
Weakly basic ion exchange resins are not ordinarily recommended for the removal of carboxylic acid. Thus, it was a most unexpected result when it was found that such resin also greatly reduced the level of bound citric acid, while at the same time greatly decreasing or virtually eliminating the objectionable bitter taste of the polydextrose.
Rennhard also suggested dialysis as a method of reducing the acidity of polydextrose. However, this method is well known to selectively remove low molecular weight compounds which diffuse through a membrane where higher molecular weight solutes do not. We now know that the citrate ester compounds (bound citric acid) which are primarily responsible for the bitterness in unimproved polydextrose span a wide range of molecular weights comparable to the molecular weight range of polydextrose itself. Thus, dialysis would be unsuitable for the removal of such compounds.